R: [visual-modflow] Re: Simulating spring flow

Hi Renè, that’s correct, I meant you can use CHD as spring if you are sure that spring will be always as sink (not source). Sometimes it’s possible, of

Message 1 of 10
, 8 Mar 2:06 am

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Hi Renè,

thats correct, I meant you can use CHD as spring if you are sure that
spring will be always as sink (not source). Sometimes its possible, of
course you cant simulate any dry condition of the spring because of the CHD
(source in this case).

You can say spring is always as sink, never as source, so, in my opinion,
you could simulate as CHD.

From my understanding, a constant head boundary can both act as a source or
sink, depending on the assigned head value; so that's why I thought it won't
be
reasonable to use this as a boundary. Can you please elaborate more on how I
can
use this constant head as a sink only (i.e. no inflow to the model).

Why don't you assign a constant head and find a k to obtain your
discharges ?

Cheers,
Daniele

Rene Pericles Mbanguka

Thanks Daniele, I will give it a try. The complication is that the springs are located somewhere inside my mode domain. From my previous experience, CHD cells

Message 2 of 10
, 8 Mar 2:37 am

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Thanks Daniele, I will give it a try. The complication is that the springs are
located somewhere inside my mode domain. From my previous experience, CHD cells
act as sinks when they located at the downstream end of the model, and they act
as sources once the are on the upstream boundary. For the case of a CHD inside
the model domain, I think you always experience "In" and "Out" components!
Regards,
René.
________________________________
Eng. René Périclès MBANGUKA
Water Res. Eng. & Modeller
WEMA Consult (T) Ltd
Dar es Salaam - TZ
Phone : +255 653 35 92 520
Alt mail: rene@...
Web link: www.wemaconsult.com

Daniele Baldi

Well, in this case you have to use a drain to simulate spring. About your problem, have you checked the head out of the drain? In order to achieve the

Message 3 of 10
, 8 Mar 6:26 am

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Well, in this case you have to use a drain to simulate spring.

About your problem, have you checked the head out of the drain? In order to
achieve the discharge, maybe you need to increase local gradient.

Thanks Daniele, I will give it a try. The complication is that the springs
are
located somewhere inside my mode domain. From my previous experience, CHD
cells
act as sinks when they located at the downstream end of the model, and they
act
as sources once the are on the upstream boundary. For the case of a CHD
inside
the model domain, I think you always experience "In" and "Out" components!
Regards,
René.
________________________________
Eng. René Périclès MBANGUKA
Water Res. Eng. & Modeller
WEMA Consult (T) Ltd
Dar es Salaam - TZ
Phone : +255 653 35 92 520
Alt mail: rene@... <mailto:rene%40wemaconsult.com>
Web link: www.wemaconsult.com

chrisli1223

Hi Rene, Daniele s CHD approach sounds reasonable to me. I understand your concern about the fact that CHD can both act as a source and sink. I would use Zone

Message 4 of 10
, 8 Mar 1:36 pm

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Hi Rene,

Daniele's CHD approach sounds reasonable to me. I understand your concern about the fact that CHD can both act as a source and sink. I would use Zone Budget to set up a zone at the CHD cells. The CHD-in value would show you only the amount of flow that goes into the CHD cells.

Also check your CHD-out value. If it is zero, then CHD is purely a sink and we do not need to worry further. Otherwise, continue to read.

If there is outflow from the CHD cells, then it is going to change the surrounding area and hence may affect the inflow to the CHD cells. Therefore you should only use the CHD-in value as your preliminary start point of your calibration.

The following is just my opinion.

Springs are simulated by drainage cells because (i) it is always a sink (ii) we do not know about the amount of spring discharge. We need drainage cells to tell us the answer.

In your case, (i) is true but (ii) is false as you already know the discharge value.

Another type of boundary condition that can fulfill (i)=true and (ii)=false is the Well package. It is always a sink. It requires known discharge rates.

Since you have assigned very high drain conductance but the discharge is still too low. Therefore the next thing I would look at will be K (higher K = transmits more water = more discharge).

I would use the Well package and vary K until the head at the wells are near the drain elevation (surface elevation of the spring). After your K is right, you can take away the wells and put the drainage cells in.

The methodology I just mentioned, however, is purely based on theory and has not been tested yet. :p

Hi Chris, Thank you very much for your suggestions. The well approach sounds very logical indeed. I tried a good number of runs but I ve just realised my

Message 5 of 10
, 9 Mar 6:10 am

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Hi Chris,

Thank you very much for your suggestions. The well approach sounds very logical
indeed. I tried a good number of runs but I've just realised my spring discharge
is perhaps too high! I had to increase K up to >100m/d which is not physically
true. K values have been approximated to be between 5 and 10m/d in the area. The
spring is actually said to be due to faults, so I think setting too high
conductivity for the whole hydrogeologic unit containing the spring is may be
not logical!
And another question is : how do I relate my estimated K value to the drain
conductance?

Daniele's CHD approach sounds reasonable to me. I understand your concern about
the fact that CHD can both act as a source and sink. I would use Zone Budget to
set up a zone at the CHD cells. The CHD-in value would show you only the amount
of flow that goes into the CHD cells.

Also check your CHD-out value. If it is zero, then CHD is purely a sink and we
do not need to worry further. Otherwise, continue to read.

If there is outflow from the CHD cells, then it is going to change the
surrounding area and hence may affect the inflow to the CHD cells. Therefore you
should only use the CHD-in value as your preliminary start point of your
calibration.

The following is just my opinion.

Springs are simulated by drainage cells because (i) it is always a sink (ii) we
do not know about the amount of spring discharge. We need drainage cells to tell
us the answer.

In your case, (i) is true but (ii) is false as you already know the discharge
value.

Another type of boundary condition that can fulfill (i)=true and (ii)=false is
the Well package. It is always a sink. It requires known discharge rates.

Since you have assigned very high drain conductance but the discharge is still
too low. Therefore the next thing I would look at will be K (higher K =
transmits more water = more discharge).

I would use the Well package and vary K until the head at the wells are near the
drain elevation (surface elevation of the spring). After your K is right, you
can take away the wells and put the drainage cells in.

The methodology I just mentioned, however, is purely based on theory and has not
been tested yet. :p

Feel free to comment.

Cheers,
Chris

Daniele Baldi

Hi Renè, if you know fault’s patters, you could apply local high hydraulic conductivity in order to simulate high fault’s k. Do you have any idea of

Message 6 of 10
, 10 Mar 11:53 am

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Hi Renè, if you know faults patters, you could apply local high hydraulic
conductivity in order to simulate high faults k.

Do you have any idea of piezometric around the spring? Are you able to find
local preferential flow way due to the faults?

Anyway, your spring discharge isnt too high, I mean, its your true data,
so you have to play with piezometric gradient, k (locally by faults line)
and (but youve just do it) spring conductance.

Thank you very much for your suggestions. The well approach sounds very
logical
indeed. I tried a good number of runs but I've just realised my spring
discharge
is perhaps too high! I had to increase K up to >100m/d which is not
physically
true. K values have been approximated to be between 5 and 10m/d in the area.
The
spring is actually said to be due to faults, so I think setting too high
conductivity for the whole hydrogeologic unit containing the spring is may
be
not logical!
And another question is : how do I relate my estimated K value to the drain
conductance?

Theoretically a spring is a drain of the system and you should calibrate the hydraulic parameter of the drain (conductance) on the base of the observations of

Message 7 of 10
, 10 Mar 9:34 pm

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Theoretically a spring is a drain of the system and you should calibrate the
hydraulic parameter of the drain (conductance) on the base of the
observations of the measured discharges. If you are not able to repreduce
the same or similar discharges it means that your conceptualization
(boundary conditions or other thins) is not OK.
Of course, for a calibration run you can use other packages like the well
package or the EVT package to simulate the springs and to force the
abstraction of the water you have observed . However for a predictive run
you have to use the drain package and if the drain conductance is not
calibrated or the conceptual model is not OK you might get wrong results.

> Hello again!
>
> How can I model spring discharge in Visual Modflow? I do understand springs
> are often simulated with the drain package, but my problem is on how to get
> the springs to deliver the observed/recorded flow! Im my model, there is a
> spring which discharges 3.5m^3/s. I tried to assign a drain channel around
> the spring area in my model, but the channel can only deliver very little
> water. I tried to raise the drain conductance very high but I still can't
> reach the recorded flow!
>
> Cheers!

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